1. Field of the Invention
This invention relates to a medical imaging apparatus and to a method for furnishing difference images. More particularly, this invention relates to a medical imaging apparatus and method, such apparatus and method having features for reducing the effect of patient motion on difference images. Preferably, this invention relates to a medical X-ray imaging apparatus and method.
2. Description of a Difference Imaging Apparatus and Method
In the commonly-owned co-pending patent application by Ivan Jazbec entitled "Medical Imaging Apparatus", Ser. No. 278,479, there is disclosed a medical imaging apparatus which detects the flow of a contrast medium through an organ of the body under study. Even a small amount of the tracer of contrast medium present in the study organ can be detected. The organ of the patient is exposed to the nuclear radiation of the tracer or to X-rays, respectively. The intensity distribution of the radiation transmitted through the organ is determined by means of a detector such as an image intensifier for X-rays. A TV camera coupled to the image intensifier issues a video signal typically in analog form. This video signal corresponds to a predetermined number of images per second. It may be digitized with respect to the intensity of the individual image elements. Finally it is displayed on a video monitor.
In particular, at least one image is usually taken prior to the injection of the contrast medium into the patient's blood stream. This image forms a reference image and is referred to as "primary mask". It is stored in a first storage device or first memory. In order to carry out the study, difference images are obtained by subtracting the primary mask from subsequent images or frames. The study is begun when the contrast medium enters the area of interest, for instance the aorta of the human heart. The flow of the contrast medium into and through the organ under examination can be observed by displaying the difference images on the display screen. Such a study can be carried out by processing live or recorded images. Thus, a replay feature may be provided. In order to make a replay possible, the images obtained after the start of the study are stored in a second storage device or second memory. From here they can be displayed on the screen (either with or without the subtraction of the primary mask) as often as required.
The disclosure of the aforementioned co-pending patent application of Ivan Jazbec, Ser. No. 278,481, is incorporated herein by reference.
Substantial time may pass between the recording of the primary mask and the appearance of the contrast medium in the difference images. During this time interval it is possible that the patient and/or organ of interest will move. This will result in a shift in the displayed image. The difference images can be degraded by these movements especially within this time interval. It is desirable to minimize or at least reduce the effect of patient motion on the difference images.